True Fusion: AMD A8-3800 APU Review. Page 5

Desktop Lynx platform that includes hybrid Llano processors has finally found its way to the consumers. Let’s take a closer look at it and find out how successful the combination of old Stars processor cores and a high-performance Radeon GPU actually is.

North Bridge

The North Bridge integrated into the processor is responsible not only for working with the memory, but also for providing PCI Express support. It means that Lynx platform has become very similar in structure to Intel systems, and the actual mainboard core logic set has degenerated into a plain South Bridge.

Compared with the previous-generation processors, Llano memory controller has become much more functional. Yes, it still supports dual-channel memory, just like before, however today it officially supports not only DDR3-1333, but also DDR3-1600/1866 SDRAM. Although when you use high-speed DDR3-1866, it will only work if you install one module per channel.

The graphics core obviously needs higher memory bus bandwidth. Even AMD themselves point out that the use of DDR3-1333 memory may significantly slow down the integrated graphics. However, high-speed memory support is not the major peculiarity of the North Bridge. The much more interesting thing about it is the new data paths, which should optimize the APU graphics performance. Besides the traditional link between the CPU and the North Bridge, Llano has two additional bus connections between the GPU and the North Bridge.

The first one called Radeon Memory Bus is used for regular graphics applications. Its bandwidth is 29.8 GBps, which is the same as the data transfer rate of DDR3-1866 SDRAM. The memory requests going along this bus have the highest priority and the memory controller processes them even ahead of the processor requests.

The second bus between the graphics core and the memory controller is called AMD Fusion Compute Link and provides graphics core with direct access to the memory at the same time ensuring processor cache coherency. In other words, Fusion Compute Link is used for direct data transfer between the graphics and computational cores when they work as a team on the same task. Today this bus is barely used, but its benefits should really start to shine later on as Fusion concept becomes more popular and a larger number of applications are able to take full advantage of the APU functionality.

As for the PCI Express bus controller, Llano supports 24 second-generation lanes. Four lanes are assigned to connect to the chipset South Bridge, another four are given to peripheral devices. The remaining 16 lanes form a PCIe x16 bus for external graphics accelerators. This bus in its turn can be split in two PCIe x8, so Lynx platform supports CrossFire not only in Dual Graphics implementation, but also in a traditional configuration with two add-on cards.